A hydrostatic power transmission which uses hydraulic pump/motors and is employed as a continuously variable transmission has been known. This transmission has the advantage that speed can be continuously varied. However, its efficiency is not always high. Further, the range of speeds is not satisfactorily wide. Accordingly, a hydromechanical transmission (HMT) has been developed and published in which such a hydrostatic power transmission (HST) is used in conjunction with differential gears and the transmission of power is shared with the HST and the differential gears so as to attain a high transmission efficiency of the gearing as well as the continuous variableness of the HST. See Chieo Ishihara: "Hydrodynamic Engineering," Asaka-shobo Japan and Sadao Ishihara: "Theory and Practice of Piston Pump Motor," Corona-sha, Japan. Furthermore a hydromechanical transmission developed by one of the same inventors of the instant invention is commonly owned and disclosed in co-pending U.S. application Ser. No. 921,099, filed on Oct. 21, 1986. Such a transmission includes a differential mechanism having first, second, third input/output ends and forming either a first mechanical transfer line for low speeds between the first and second input/output ends or a second mechanical transfer line for high speeds between the first and third input/output ends; a hydraulic power transmission mechanism having two hydraulic pump/motors one of which acts as a hydraulic pump when the other functions as a hydraulic motor, the input/output shaft of one of the pump/motors being connected to the second input/output end of the differential mechanism, the input/output shaft of the other pump/motor being connected to the third input/output shaft, the two pump/motors cooperating to constitute continuous variable hydraulic power transmission lines; a low-speed clutch for bringing the transmission end of the first mechanical transfer line into and out of engagement with a common rotary element disposed on the input or output side; and a high-speed clutch for bringing the transmission end of the second mechanical transfer line into and out of engagement with the common rotary element. When one of the clutches is engaged, the other is disengaged and vice versa to select either the low-speed or high-speed mode.
In the above structure, when it is switched from one mode to the other, a shock tends to occur. Specifically, in this transmission, a portion of the applied power is transmitted toward the output via one mechanical transfer line including gears. The remaining power is directed toward the output via the hydraulic power transmission line including pump/motors, so that this power is added to the power which is transmitted by the mechanical transfer line. When a load is applied to the hydraulic transmission line, rotary output from this line tends to lag the output from the mechanical transfer line, because of leakage of the working fluid inside the pump/motors and because of the effects of the elasticity of the hydraulic circuit. In this conventional structure, when it is switched from one mode to the other, the roles of the two pump/motors, i.e., the pump/motors, are interchanged as described later. For this reason, the effect of the aforementioned lag is exerted in reverse. Therefore, when the transmission is switched from the low-speed mode to the high-speed mode, for example, if the low-speed clutch is disengaged and, at the same time, the high-speed clutch is engaged just when the rotational speed at the transmission end of the second mechanical transfer line becomes equal to the rotational speed at the transmission end of the first mechanical transfer line or after a certain period elapses since the coincidence of the speeds, then the transmission of torque from the first mechanical transfer line to the common rotary element will be suddenly interrupted. This causes an indeterminate idling region that might be called a backlash of the whole system to be momentarily absorbed. Then, torque is transmitted from the second mechanical transfer line to the common rotary element. Consequently, a large mechanical shock often occurs momentarily when such a switching operation is performed.